Coherent light illuminating a rough surface produces speckle. Reflection from the rough surface is referred to as diffuse reflection. Transmission through the rough surface is referred to as diffuse transmission. In the diffuse reflection or the diffuse transmission, light scatters in various directions. The coherent light scattered by the diffuse reflection or by the diffuse transmission forms an interference pattern in the space away from the rough surface. If viewed by a human eye, the eye will see dark and light in a ‘granular’ pattern. The granular pattern is the speckle. An intensity detector of an optical system will also detect the speckle if the optical system views the rough surface illuminated by the coherent light.
A first speckle demonstration apparatus of the prior art is illustrated in FIG. 1. The first speckle demonstration apparatus 1 includes a first demonstration laser 2, a first diverging lens 4, and a first viewing screen 6, which are located on a first optic axis 8. The first demonstration laser 2 emits a first laser beam 10. The first diverging lens 4 transforms the first laser beam 10 into a divergent laser beam 12. The divergent laser beam 12 illuminates the first viewing screen 6 in a first large area 14. The first viewing screen 6 diffusely reflects the divergent laser beam 12 creating an interference pattern. An observation plane 16 located on a second optic axis 18 intersects the interference pattern. The observation plane 16 is the field-of-view in space where the eye or the optical system is focused. Note that the diverging lens 4 aids in demonstrating the speckle but is not necessary to produce the speckle.
FIG. 2 is a photograph of a typical speckle pattern 17 of the prior art, which is illustrative of the speckle viewed at the observation plane 16. Constructive interference of the divergent laser beam 12 reflecting diffusely from the viewing screen 6 creates bright spots in the observation plane 16. Destructive interference creates dark spots between the bright spots. The diffuse reflection from the viewing screen 6 has a random nature so the bright spots and the dark spots vary throughout the observation plane 16.
A measure of the speckle is contrast (C). The contrast, in percent, is given by C=100*IRMS/Ī where Ī is a mean intensity and IRMS is a root mean square intensity fluctuation about the mean intensity.
Goodman in “Some fundamental properties of speckle” (J. Opt. Soc. A., vol. 66, no. 11, November 1976, pp 1145–1150) teaches that the speckle can be reduced by superimposing N uncorrelated speckle patterns. This reduces the contrast by 1/√{square root over (N)} provided that the N uncorrelated speckle patterns have equal mean intensities and contrasts. If the N uncorrelated speckle patterns have non-equal mean intensities or non-equal contrasts, the speckle reduction factor will be greater than 1/√{square root over (N)}. Thus, the 1/√{square root over (N)} reduction factor is a best case for the speckle reduction for the N uncorrelated speckle patterns.
Goodman further teaches that the uncorrelated speckle patterns can be obtained by means of time, space, frequency, or polarization. For example, the space means could be produced by a second demonstration laser, operating at the same wavelength as the first demonstration laser 2, and a second diverging lens located on a third optic axis that illuminates the first large area 14 of the first viewing screen 6. Provided that the first optic axis 8 and the third optic axis are separated by a sufficient angle, the speckle will be reduced by 1/√{square root over (2)}. Angular separation is necessary because, if the second demonstration laser having a second laser beam is configured such that the first laser beam 10 and the second laser beam coincide, there will be no reduction in the speckle provided that the time, frequency, or polarization means are not employed. This is because the first demonstration laser 2 and the second demonstration laser produce the same speckle patterns when the angular separation is not present. This is despite the fact that the first demonstration laser 2 is incoherent with the second demonstration laser.
Goodman further teaches that the polarization means could be a depolarizing screen, which reflects or transmits polarized light as randomly polarized diffuse light. The speckle pattern produced by the depolarizing screen differs significantly if viewed through a polarization analyzer while rotating the polarization analyzer. This indicates that two orthogonal polarization components illuminating the depolarizing screen produce two uncorrelated speckle patterns. Thus, if the viewing screen 6 is a 100% depolarizing screen, the contrast is reduced by 1/√{square root over (2)}.
Another method known in the art for creating multiple speckle patterns is to move the viewing screen 6 in an oscillatory motion 19, which employs the time means taught by Goodman. The oscillatory motion 19 typically follows a small circle or a small ellipse about the optic axis 8. This causes the speckle pattern to shift relative to the eye or the optical system viewing the viewing screen and, thus, forms multiple speckle patterns over time. Though the amount of the speckle at any instant in time is unchanged, the eye perceives the reduced speckle provided that the speed of the oscillatory motion is above a threshold speed. The intensity detector of the optical system detects the reduced speckle provided that an exposure time is sufficiently long to allow the speckle pattern to move a significant distance.
A second speckle demonstration apparatus of the prior art is illustrated in FIG. 3. The second speckle demonstration apparatus 20 includes a third demonstration laser 22, a cylindrical divergent lens 24, a scanning mirror 26, and a second viewing screen 28. The third demonstration laser 22 emits a third laser beam 30, which is coupled to the cylindrical divergent lens 24. The cylindrical divergent lens 24 transforms the third laser beam into a second divergent laser beam 32. The scanning mirror 26 reflects the second divergent laser beam 32. Thus, the second divergent laser beam 32 forms a line illumination 33 on the second viewing screen 28. The scanning mirror 26 repeatedly scans the line illumination 33 across a portion of the second viewing screen 28 with a scanning motion 34 having a scanning frequency. Thus, a second large area 36 is illuminated. If the eye or the optical system views the second viewing screen 28, the eye or the intensity detector will detect illumination across the second viewing screen 28 provided that the scanning frequency is sufficiently high. The eye or the intensity detector will also detect the speckle.
Speckle is a considerable problem in many laser illuminated systems. This difficulty is exacerbated by a relatively slow reduction in the contrast due to the 1/√{square root over (N)} reduction factor. What is needed are additional methods of reducing laser speckle.